Optical communication systems are a substantial and fast-growing constituent of communication networks. The expression "optical communication system," as used herein, relates to any system which uses optical signals to convey information across an optical waveguiding medium. Such optical systems include, but are not limited to, telecommunications systems, cable television systems, and local area networks (LANs). Optical systems are described in Gowar, Ed. Optical Communication Systems, (Prentice Hall, N.Y.) c. 1993, the disclosure of which is incorporated herein by reference. Currently, the majority of optical communication systems are configured to carry an optical channel of a single wavelength over one or more optical waveguides. To convey information from plural sources, time-division multiplexing is frequently employed (TDM). In time-division multiplexing, a particular time slot is assigned to each signal source, the complete signal being constructed from the portions of the signals collected from each time slot. While this is a useful technique for carrying plural information sources on a single channel, its capacity is limited by fiber dispersion and the need to generate high peak power pulses.
While the need for communication services increases, the current capacity of existing waveguiding media is limited. Although capacity may be expanded, e.g., by laying more fiber optic cables, the cost of such expansion is prohibitive. Consequently, there exists a need for a cost-effective way to increase the capacity of existing optical waveguides.
Wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of existing fiber optic networks. A WDM system employs plural optical signal channels, each channel being assigned a particular channel wavelength. In a WDM system, signal channels are generated, multiplexed, transmitted over a single waveguide, and demultiplexed to individually route each channel wavelength to a designated receiver. Through the use of optical amplifiers, such as doped fiber amplifiers, plural optical channels are directly amplified simultaneously, facilitating the use of WDM systems in long-distance optical systems.
Proposed wavelength division multiplexed optical communication systems typically include multiplexer and demultiplexer designs which permit only a fixed number of optical channels to be used in the optical system. In one optical system configuration, the multiplexed signal is broken down into its constituent optical signals through the use of an integrated frequency router demultiplexer. The frequency router uses silicon optical bench technology in which plural phosphorus-doped silica waveguides are disposed on a silicon substrate. An optical star outputs to an array of N waveguides having adjacent optical path lengths which differ by q wavelengths; this array in turn feeds an output N.times.N star. This frequency router design for an optical communication system is described in Alexander et al., J. Lightwave Tech., Vol. 11, No. 5/6, May/June 1993, p. 714, the disclosure of which is incorporated herein by reference. Using a 1.times.N configuration at the input, a multiplexed optical signal containing light of different frequencies is separated into its component frequencies at each waveguide extending from the output N.times.N star. Although this configuration adequately separates light of different frequencies, the integrated optical design fixes both the number and the respective frequencies of the optical channels. Consequently, adding or decreasing the number of optical channels or changing the channel wavelength or spacing is not possible without providing a completely new frequency router demultiplexer to the optical system.
Other proposed WDM optical communication systems employ gratings-based demultiplexers in which Bragg gratings are used to create a transmission filter having a passband corresponding to the selected optical channel. Such a demultiplexer for a four-channel WDM system is depicted in Mizrahi et al., Electronics Letters, Vol. 30, No. 10, May, 1994, p. 780, the disclosure of which is incorporated herein by reference. A 1.times.4 fused fiber splitter has a fiber transmission filter spliced to each of its four output ports. Two Bragg gratings are written into the filters which pass the desired channel wavelength and which reject the other three channel wavelengths. While this design is acceptable for a limited number of fixed optical channels, transmission-based filters must be designed to reject all non-selected channels for each demultiplexer output; their use therefore requires advance knowledge of all system channels and their respective wavelengths. Consequently, the described gratings-based transmission filter demultiplexer does not facilitate expansion of the WDM system with additional optical channel wavelengths.
Thus, there is a need in the art for improved wavelength division multiplexed optical communication systems which are configured to carry varying quantities of optical signal channels at various channel frequencies. Such WDM optical communication systems would be readily expandable for carrying increased numbers of optical channels when there is a need for greater system capacity.